I don't have access to machine/drive, but need to define specs to purchase one.

What all things would it be dependent on? Using RAID 10, would improve it?

Would it be dependent on file size, which we read or write?

How to calculate latency of fetching a file segment? Seek time etc..

I need to use it with a web server, for storing and delivering large no. of small files. There will be continuous writes and reads 24x7 of small files. Any recommendations for which RAID to use, or no RAID should be used, based on writes or reads?

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I'm not going to answer, but you should bear this in mind. 7200 disks are slooow - if you can afford it, get 10k or 15k. You should also bear in mind larger capacity disks will take longer to rebuild in the event of a disk failure, thus increasing your risk of double disk failure.
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Ben PilbrowOct 1 '11 at 10:15

@BenPilbrow Yeah, but 10k, 15k disks are too expensive. And I need atleast about 8TB of storage, on server.
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NagotaOct 1 '11 at 10:38

1 Answer
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Using RAID will improve it, but only if you choose an appropriate stripe size, if you have several small files, a smaller stripe size is recommended.
And I think the only way to calculate is to put it to the test, every case is different.